Formate lactose glutamate: A chemically defined medium as a possible substitute for MacConkey broth in the presumptive coliform examination of water

Abstract

During a period of 13 months, 1273 consecutive samples of water received at this laboratory were submitted to a modified presumptive coliform test which included single tubes of four different media. These were: (1) MacConkey broth, (2) formate lactose glutamate medium (pH 7·5), (3) lactose glutamate medium (pH 7·5), and (4) Folpmers's glucose glutamic acid (pH 6·0). 955 samples gave exactly the same results in all four media, but the remaining 318 samples produced differences which enabled comparisons to be made between the media.As compared with MacConkey broth, the three glutamic-acid media produced between 24–30% more isolations ofEsch. coli, and the formate and the glucose media produced at least the same total number of true coliform organisms (includingEsch. coli) with appreciably fewer false positive reactions. The lactose glutamate medium gave no false positive reactions but, through the suppression of coliform organisms other thanEsch. coli, reduced by 16% the total coliform yield. MacConkey broth gave the largest early (18 and 24 hr.) yield of positive reactions, but the results at the end of 24 hr. showed the formate medium to be not far behind MacConkey broth and appreciably ahead of the glucose medium.Further experiments with the formate lactose glutamate medium adjusted to different pH's, ranging from 6·0–7·5, indicated that a medium of pH 6·7 provided optimal conditions for the early development of both acid and gas. It was therefore decided to test this observation more fully with routine samples. During a period of 4 months, 279 unselected consecutive samples of water (of which 78 gave positive results) and 57 consecutive unchlorinated water samples (56 of which gave positive results) were examined by a multiple-tube technique in both MacConkey broth and formate lactose glutamate medium (pH 6·7). In the former series the MacConkey broth contained the inhibitory bile salt previously used; in the latter series this was replaced by a relatively non-inhibitory bile salt. In both series the formate medium yielded an appreciably increased total of coliform organisms (12 and 17%) including an increased total ofEsch. coli(27% in the former and 13% in the latter series), and in both series also the results at 18 hr. were abreast with, and at 24 hr. ahead of, those obtained in MacConkey broth.With certain reservations, it is considered that formate lactose glutamate medium (pH 6·7) can be offered as a suitable alternative to MacConkey broth for the presumptive coliform test of water. Wider trials of this medium would be required before this claim could be fully substantiated.My thanks are gratefully accorded to Drs W. H. H. Jebb and A. H. Tomlinson of the Oxford Public Health Laboratory for invaluable advice during the investigation and to Dr Ian Sutherland of the M.R.C. Statistical Research Unit for statistic appraisal of the results. I am also greatly indebted to my technicians Mr J. H. Evans, F.I.M.L.T., and Mr G. H. Lowe, F.I.M.L.T., for their unstinting help.